NCERT Summary: Summary of Physics- 3 UPSC Notes | EduRev

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MECHANICS

  • Motion: In physics, motion is change of location or position of an object with respect to time. Mechanical motion is of two types, transitional (linear) and rotational (spin).
  • SPEED: The speed of a moving body is the rate at which it covers distance i.e. the distance it covers per unit of time.

Speed: (distance travelled/ time required.) The S.I. Unit of speed is ms.

VELOCITY: The distance covered by an object in a specified direction in unit time interval is called velocity. The S.I. Unit of velocity is m/s.

  • Average velocity can be calculated by dividing displacement over time.
  • The instantaneous velocity shows the velocity of an object at one point.
  • The difference between speed and velocity is: Speed is the distance travelled by an object in a particular time. Velocity is the speed in a particular direction.
  • ACCELERATION: When an object’s velocity changes, it accelerates. Acceleration shows the change in velocity in a unit time. Velocity is measured in meters per second, m/s, so acceleration is measured in (m/s)/ s, or m/s2, which can be both positive and negative. The symbol for acceleration is a (boldface).
  • When the velocity decreases the body is said to undergo retardation or deceleration.
  • Acceleration Due to Gravity: Galileo was the first to find out that all objects falling to Earth have a constant acceleration of 9.80 m/s2 regardless of their mass. Acceleration due to gravity is given a symbol g, which equals to 9.80 m/s2.
  • FORCE: Force can be defined as a push or a pull. (Technically, force is something that can accelerate objects.) Force is measured by N (Newton). A force that causes an object with a mass of 1 kg to accelerate at 1 m/s is equivalent to 1 Newton.
  • Newton’s law of universal gravitation states that every massive particle in the universe attracts every other massive particle with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
  • In equation form, the gravitational force F = G(m1 m2)/ r2 where r is the distance between two bodies of masses mand m2 and G the universal gravitational constant.
  • Centripetal Force: For a body to move in a circle there must be a force on it directed towards the centre. This is called the centripetal force and is necessary to produce continuous change of direction in a circular motion.
  • The magnitude of the centripetal force on an object of mass m moving at a speed v along a path with radius of curvature r is given by the relation F =mv2/r The direction of the force is toward the center of the circle in which the object is moving. Centrifugal force is equal and opposite to centripetal force, i.e it acts outwards.
  • WEIGHT: the weight of a body is the force with which the earth attracts the body towards its centre. The weight of a body should not be confused with its mass, which is a measure of the quantity of matter contained in it. Mass shows the quantity, and weight shows the size of gravity. The weight of a body is maximum at the poles and minimum at equator.
  • If you know your mass, you can easily find your weight because W = mg where:

W is weight in Newton (N),
m is mass in kg, and
g is the acceleration of gravity in m/s2.

  • Weight is measured by Newton (N).
  • It is now obvious that the value of g is maximum at poles and minimum at equator. At the centre of earth, g would be zero.
  • It should be noted here that on the surface of the moon the value of the acceleration due to gravity is neraly one-sixth of that on earth, and therefore, an object on the moon would weigh only one-sixth its weight on earth.
  • Newton’s Laws of Motion:

1. Newtons First Law of Motion:

  • Newton’s first law of motion states that “An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.” . Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.
  • In fact, it is the natural tendency of objects to resist changes in their state of motion. This tendency to resist changes in their state of motion is described as inertia.
  • Inertia: Inertia is the tendency of an object to resist changes in its state of motion. But what is meant by the phrase state of motion? The state of motion of an object is defined by its velocity - the speed with a direction. Thus, inertia could be redefined as follows: Inertia: tendency of an object to resist changes in its velocity.
  • There are many more applications of Newton’s first law of motion.
  • Blood rushes from your head to your feet while quickly stopping when riding on a descending elevator.
  • The head of a hammer can be tightened onto the wooden handle by banging the bottom of the handle against a hard surface.
  • While riding a skateboard (or wagon or bicycle), you fly forward off the board when hitting a curb or rock or other object which abruptly halts the motion of the skateboard.

2. Newton’s Second Law of Motion: 

  • The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.
  • The relationship between an object’s mass m, its acceleration a, and the applied force F is F = ma. Acceleration and force are vectors (as indicated by their symbols being displayed in slant bold font); in this law the direction of the force vector is the same as the direction of the acceleration vector.

3. Newton’s Third Law of Motion:

  • For every action, there is an equal and opposite reaction.
  • The statement means that in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second object. The direction of the force on the first object is opposite to the direction of the force on the second object. Forces always come in pairs - equal and opposite action-reaction force pairs.
  • The rocket’s action is to push down on the ground with the force of its powerful engines, and the reaction is that the ground pushes the rocket upwards with an equal force.
  • There’s also the example of shooting a cannonball. When the cannonball is fired through the air (by the explosion), the cannon is pushed backward. The force pushing the ball out was equal to the force pushing the cannon back, but the effect on the cannon is less noticeable because it has a much larger mass. That example is similar to the kick when a gun fires a bullet forward.
  • Friction: Friction is a force that resists the movement oof one surface over another. The force acts in the opposite direction to the way an object wants to slide. If a car needs to stop at a stop sign, it slows because of the friction between the brakes and the wheels.
  • Measures of friction are based on the type of materials that are in contact. Concrete on concrete has a very high coefficient of friction. That coefficient is a measure of how easily one object moves in relationship to another. When you have a high coefficient of friction, you have a lot of friction between the materials.
PROPERTIES OF MATTERS
  • Properties of matters: A matter can neither be created nor it can be destroyed but it can be transformed from one state to another. Matter is made of basic building blocks commonly called elements which are 112 in number. The matter is made of only one kind of element then the smallest unit of that element is called an atom. If the matter is made of two or more different elements then the smallest unit of matter is called a molecule.
  • Molecule is defined as the smallest unit of matter which has independent existence and can retain complete physical and chemical properties of matters.
  • According to kinetic theory of matter:
    (i) molecules are in the state of continuous motion in all possible directions and hence they posses  kinetic energy which increases with the gain of heat energy or rise in temperature,
    (ii). the molecules always attract each other,
    (iii). the force of attraction between the molecules decreases with the increase in intermolecular spaces
  • The molecules always attract each other. The force of attraction between the similar kind of molecules is called force of cohesion whereas the force of attraction between different kinds of molecules is called force of adhesion.
  • In case of solids, the intermolecular space being very small, so intermolecular forces are very large and hence solids have definite size and shape.
  • In case of liquids, the intermolecular space being large, so intermolecular forces are small and hence liquids have definite volume but no definite shape.
  • In case of gases, the intermolecular space being very large, so intermolecular forces are extremely small and hence gases have neither a definite volume and nor definite shape.
  • A solid has definite shape and size. In order to change (or deform) the shape or size of a body, a force is required. If you stretch a helical spring by gently pulling its ends, the length of the spring increases slightly. When you leave the ends of the spring, it regains its original size and shape. The property of a body, by virtue of which it tends to regain its original size and shape when the applied force is removed, is known as elasticity and the deformation caused is known as elastic deformation.
  • However, if you apply force to a lump of putty or mud, they have no gross tendency to regain their previous shape, and they get permanently deformed. Such substances are called plastic and this property is called plasticity. Putty and mud are close to ideal plastics.
  • When a force is applied on body, it is deformed to a small or large extent depending upon the nature of the material of the body and the magnitude of the deforming force. The deformation may not be noticeable visually in many materials but it is there. When a body is subjected to a deforming force, a restoring force is developed in the body. This restoring force is equal in magnitude but opposite in direction to the applied force. The restoring force per unit area is known as stress. If F is the force applied and A is the area of cross section of the body, Magnitude of the stress = F/A. The SI unit of stress is N m–2 or pascal (Pa). Stress is the restoring force per unit area and strain is the fractional change in dimension.
  • HOOKE’S LAW: Robert Hooke, an English physicist (1635 - 1703 A.D) performed experiments on springs and found that the elongation (change in the length) produced in a body is proportional to the applied force or load. In 1676, he presented his law of elasticity, now called Hooke’s law. For small deformations the stress and strain are proportional to each other. This is known as Hooke’s law. Thus, stress ”” strain or stress = k X strain , where k is the proportionality constant and is known as modulus of elasticity.
  • The basic property of a fluid is that it can flow. The fluid does not have any  resistance to change of its shape. Thus, the shape of a fluid is governed by the shape of its container. A liquid is incompressible and has a free surface of its own. A gas is compressible and it expands to occupy all the space available to it.
  • Pascal’s Law: The French scientist Blaise Pascal observed that the pressure in a fluid at rest is the same at all points if they are at the same height, distributed uniformly throughout. We can say whenever external pressure is applied on any part of a fluid contained in a vessel, it is transmitted undiminished and equally in all directions. This is the Pascal’s law for transmission of fluid pressure and has many applications in daily life. A number of devices such as hydraulic lift and hydraulic brakes are based on the Pascal’s law.
  • The flow of the fluid is said to be steady if at any given point, the velocity of each passing fluid particle remains constant in time. The path taken by a fluid particle under a steady flow is a streamline.
  • Bernoulli’s principle states when a fluid flows from one place to another without friction, its total energy (kinetic + potential + pressure) remains constant.
  • You must have noticed that, oil and water do not mix; water wets you and me but not ducks; mercury does not wet glass but water sticks to it, oil rises up a cotton wick, inspite of gravity, Sap and water rise up to the top of the leaves of the tree, hairs of a paint brush do not cling together when dry and even when dipped in water but form a fine tip when taken out of it. All these and many more such experiences are related with the free surfaces of liquids. As liquids have no definite shape but have a definite volume, they acquire a free surface when poured in a container. These surfaces possess some additional energy. This phenomenon is known as surface tension and it is concerned with only liquid as gases do not have free surfaces. Mathematically, surface tension is defined as the force acting per unit length of an imaginary line drawn on the free surface of the liquid. The surface tension is expressed in newton/meter.
  • Most of the fluids are not ideal ones and offer some resistance to motion. This resistance to fluid motion is like an internal friction analogous to friction when a solid moves on a surface. It is called viscosity.
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